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                    <h1 class="description center-align post-title">ChannelHandler 与 ChannelPipeline 详解</h1>
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                <p>我们在上一章研究的 <code>bytebuf</code> 是一个容器用来“包装”数据。在<strong>本章我们将探讨这些容器如何通过应用程序来移动，传入和传出，以及他们的内容是如何处理的</strong>。</p>
<p>本章主要内容</p>
<ul>
<li><code>Channel</code></li>
<li><code>ChannelHandler</code></li>
<li><code>ChannePipeline</code></li>
<li><code>ChannelHandlerContext</code></li>
</ul>
<h2 id="1-ChannelHandler-家族"><a href="#1-ChannelHandler-家族" class="headerlink" title="1. ChannelHandler 家族"></a>1. ChannelHandler 家族</h2><h3 id="①-Channel-生命周期"><a href="#①-Channel-生命周期" class="headerlink" title="① Channel 生命周期"></a>① Channel 生命周期</h3><p>在 <code>Channel</code> 的生命周期中，它的状态与 <code>ChannelHandler</code> 是密切相关的，下列是 <code>Channel</code> 组件的四个状态：</p>
<table>
<thead>
<tr>
<th align="left">状态</th>
<th align="left">描述</th>
</tr>
</thead>
<tbody><tr>
<td align="left">ChannelUnregistered</td>
<td align="left">Channel 没有注册到 EventLoop</td>
</tr>
<tr>
<td align="left">ChannelRegistered</td>
<td align="left">Channel 被注册到了 EventLoop</td>
</tr>
<tr>
<td align="left">ChannelActive</td>
<td align="left">Channel 已经连接到它的远程节点，处于活动状态，可以收发数据</td>
</tr>
<tr>
<td align="left">ChannelInactive</td>
<td align="left">Channel 与远程节点断开不再处于活动状态</td>
</tr>
</tbody></table>
<p>Channel 的生命周期如下图所示，<strong>当这些状态发生改变时，将会生成对应的事件，<code>ChannelPipeline </code>中的<code>ChannelHandler </code>就可以及时做出处理</strong>：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115607525.png" alt="image-20220327115607525" style="zoom:80%;" />

<h3 id="②-ChannelHandler-生命周期"><a href="#②-ChannelHandler-生命周期" class="headerlink" title="② ChannelHandler 生命周期"></a>② ChannelHandler 生命周期</h3><p><code>ChannelHandler</code> 接口定义了其生命周期中的操作，当<code>ChanelHandler</code>被添加到<code>ChannelPipeline </code>或从<code>ChannelPipeline</code>中移除时，会调用这些操作，<code>ChannelHandler</code>的生命周期如下：</p>
<table>
<thead>
<tr>
<th align="left">方法</th>
<th align="left">描述</th>
</tr>
</thead>
<tbody><tr>
<td align="left"><code>handlerAdded</code></td>
<td align="left">当把 ChannelHandler 添加到 ChannelPipeline 中时调用此方法</td>
</tr>
<tr>
<td align="left"><code>handlerRemoved</code></td>
<td align="left">当把 ChannelHandler 从 ChannelPipeline 中移除的时候会调用此方法</td>
</tr>
<tr>
<td align="left"><code>exceptionCaught</code></td>
<td align="left">当 ChannelHandler 在处理数据的过程中发生异常时会调用此方法</td>
</tr>
</tbody></table>
<h3 id="③-ChannelHanler-子接口"><a href="#③-ChannelHanler-子接口" class="headerlink" title="③  ChannelHanler 子接口"></a>③  ChannelHanler 子接口</h3><p>Netty 提供2个重要的 ChannelHandler 子接口：</p>
<ul>
<li><code>ChannelInboundHandler</code> - 处理进站数据，并且所有状态都更改</li>
<li><code>ChannelOutboundHandler </code>- 处理出站数据，允许拦截各种操作</li>
</ul>
<h4 id="Ⅰ-ChannelInboundHandler-接口"><a href="#Ⅰ-ChannelInboundHandler-接口" class="headerlink" title="Ⅰ ChannelInboundHandler 接口"></a>Ⅰ ChannelInboundHandler 接口</h4><p><code>ChannelInboundHandler</code>会在接受数据或者其对应的<code>Channel</code>状态发生改变时调用其生命周期的方法， <code>ChannelInboundHandler</code>的生命周期和<code>Channel</code>的生命周期其实是密切相关的。 以下是<code>ChannelInboundHandler</code>的生命周期方法：</p>
<table>
<thead>
<tr>
<th align="left">方法</th>
<th align="left">描述</th>
</tr>
</thead>
<tbody><tr>
<td align="left"><code>ChannelRegistered</code></td>
<td align="left">当Channel被注册到EventLoop且能够处理IO事件时会调用此方法</td>
</tr>
<tr>
<td align="left"><code>ChannelUnregistered</code></td>
<td align="left">当Channel从EventLoop注销且无法处理任何IO事件时会调用此方法</td>
</tr>
<tr>
<td align="left"><code>ChannelActive</code></td>
<td align="left">当Channel已经连接到远程节点(或者已绑定本地address)且处于活动状态时会调用此方法</td>
</tr>
<tr>
<td align="left"><code>ChannelInactive</code></td>
<td align="left">当Channel与远程节点断开，不再处于活动状态时调用此方法</td>
</tr>
<tr>
<td align="left"><code>ChannelReadComplete</code></td>
<td align="left">当Channel的某一个读操作完成时调用此方法</td>
</tr>
<tr>
<td align="left"><code>ChannelRead</code></td>
<td align="left">当Channel有数据可读时调用此方法</td>
</tr>
<tr>
<td align="left"><code>ChannelWritabilityChanged</code></td>
<td align="left">当Channel的可写状态发生改变时调用此方法，可以调用Channel的isWritable方法检测Channel的可写性，还可以通过ChannelConfig来配置write操作相关的属性</td>
</tr>
<tr>
<td align="left"><code>userEventTriggered</code></td>
<td align="left">当ChannelInboundHandler的fireUserEventTriggered方法被调用时才调用此方法。</td>
</tr>
</tbody></table>
<p><strong>这里有一个细节一定需要注意：当我们实现<code>ChannelInboundHandler</code>的<code>channelRead</code>方法时，请一定要记住 使用<code>ReferenceCountUtil</code>的<code>release</code>方法释放<code>ByteBuf</code>，这样可以减少内存的消耗，所以我们可以实现一个 <code>ChannelHandler</code>来完成对<code>ByteBuf</code>的释放，就像下面这样：</strong></p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115615650.png" alt="image-20220327115615650" style="zoom:80%;" />

<p><strong>由于手工管理资源会很繁琐,您可以通过使用 <code>SimpleChannelInboundHandler</code> 简化问题</strong>，因为<code>SimpleChannelInboundHandler</code>已经帮我们 把与业务无关的逻辑在<code>ChannelRead</code>方法实现了，<strong>我们只需要实现它的<code>channelRead0</code>方法来完成我们的逻辑就够了</strong>：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token annotation punctuation">@ChannelHandler.Sharable</span>
<span class="token comment">// 继承 SimpleChannelInboundHandler</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">SimpleDiscardHandler</span> <span class="token keyword">extends</span> <span class="token class-name">SimpleChannelInboundHandler</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Object</span><span class="token punctuation">></span></span> <span class="token punctuation">&#123;</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">channelRead0</span><span class="token punctuation">(</span><span class="token class-name">ChannelHandlerContext</span> ctx<span class="token punctuation">,</span> <span class="token class-name">Object</span> msg<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token comment">// 不需做特别的释放资源的动作</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>💡 注意：**<code>SimpleChannelInboundHandler&lt;Object&gt;</code> 中的泛型表示要处理的入站数据的类型**</p>
<h5 id="ChannelInitializer"><a href="#ChannelInitializer" class="headerlink" title="ChannelInitializer"></a>ChannelInitializer</h5><p>回顾一下我们在第一个 Netty 应用章节的服务端代码：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">HelloServer</span> <span class="token punctuation">&#123;</span>

    <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>

    <span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">InterruptedException</span> <span class="token punctuation">&#123;</span>
        <span class="token comment">// 1.bossGroup 线程用于接收连接，workerGroup 线程用于具体的处理</span>
        <span class="token class-name">EventLoopGroup</span> bossGroup <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">NioEventLoopGroup</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">EventLoopGroup</span> workerGroup <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">NioEventLoopGroup</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">try</span> <span class="token punctuation">&#123;</span>
            <span class="token comment">// 2.创建服务端启动引导/辅助类：ServerBootstrap</span>
            <span class="token class-name">ServerBootstrap</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ServerBootstrap</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">// 3.给引导类配置两大线程组,确定了线程模型</span>
            b<span class="token punctuation">.</span><span class="token function">group</span><span class="token punctuation">(</span>bossGroup<span class="token punctuation">,</span> workerGroup<span class="token punctuation">)</span>
                    <span class="token comment">// (非必备)打印日志</span>
                    <span class="token punctuation">.</span><span class="token function">handler</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">LoggingHandler</span><span class="token punctuation">(</span><span class="token class-name">LogLevel</span><span class="token punctuation">.</span>INFO<span class="token punctuation">)</span><span class="token punctuation">)</span>
                    <span class="token comment">// 4.指定 IO 模型为 NIO</span>
                    <span class="token punctuation">.</span><span class="token function">channel</span><span class="token punctuation">(</span><span class="token class-name">NioServerSocketChannel</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span>
                    <span class="token punctuation">.</span><span class="token function">childHandler</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ChannelInitializer</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">SocketChannel</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
                        <span class="token annotation punctuation">@Override</span>
                        <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">initChannel</span><span class="token punctuation">(</span><span class="token class-name">SocketChannel</span> ch<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
                            <span class="token class-name">ChannelPipeline</span> p <span class="token operator">=</span> ch<span class="token punctuation">.</span><span class="token function">pipeline</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                            <span class="token comment">// 5.可以自定义客户端消息的业务处理逻辑</span>
                            p<span class="token punctuation">.</span><span class="token function">addLast</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">HelloServerHandler</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                        <span class="token punctuation">&#125;</span>
                    <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>
    <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>其中加入 <code>ServerBootstrap</code> 中处理的 channel 是一个 <code>ChannelInitializer </code>，这是怎么回事呢，不是应该相应的<code>ChannelHandler</code> 吗？我们来看一下<code>Channelnitializer</code>源代码：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">ChannelInitializer</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">C</span> <span class="token keyword">extends</span> <span class="token class-name">Channel</span><span class="token punctuation">></span></span> <span class="token keyword">extends</span> <span class="token class-name">ChannelInboundHandlerAdapter</span> <span class="token punctuation">&#123;</span>
 
    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">InternalLogger</span> logger <span class="token operator">=</span> <span class="token class-name">InternalLoggerFactory</span><span class="token punctuation">.</span><span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token class-name">ChannelInitializer</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
 
    <span class="token comment">/**
     * 这个方法会在Channle被注册时候调用，在方法返回之后，这个实例会在Channel对应的ChannelPipeline中删除
     *
     * @param ch            the &#123;@link Channel&#125; which was registered.
     * @throws Exception    is thrown if an error occurs. In that case the &#123;@link Channel&#125; will be closed.
     */</span>
    <span class="token keyword">protected</span> <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">initChannel</span><span class="token punctuation">(</span><span class="token class-name">C</span> ch<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">Exception</span><span class="token punctuation">;</span>
 
    <span class="token annotation punctuation">@SuppressWarnings</span><span class="token punctuation">(</span><span class="token string">"unchecked"</span><span class="token punctuation">)</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">void</span> <span class="token function">channelRegistered</span><span class="token punctuation">(</span><span class="token class-name">ChannelHandlerContext</span> ctx<span class="token punctuation">)</span>
            <span class="token keyword">throws</span> <span class="token class-name">Exception</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">boolean</span> removed <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
        <span class="token keyword">boolean</span> success <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
        <span class="token keyword">try</span> <span class="token punctuation">&#123;</span>
            <span class="token function">initChannel</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token class-name">C</span><span class="token punctuation">)</span> ctx<span class="token punctuation">.</span><span class="token function">channel</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            ctx<span class="token punctuation">.</span><span class="token function">pipeline</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            removed <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
            ctx<span class="token punctuation">.</span><span class="token function">fireChannelRegistered</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            success <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Throwable</span> t<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
            logger<span class="token punctuation">.</span><span class="token function">warn</span><span class="token punctuation">(</span><span class="token string">"Failed to initialize a channel. Closing: "</span> <span class="token operator">+</span> ctx<span class="token punctuation">.</span><span class="token function">channel</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> t<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span> <span class="token keyword">finally</span> <span class="token punctuation">&#123;</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>removed<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
                ctx<span class="token punctuation">.</span><span class="token function">pipeline</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">&#125;</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>success<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
                ctx<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">&#125;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>从上面可以看出**<code>ChannelInitializer</code>其实也是一个<code>ChannelHandler</code>**，只是<code>ChannelInitializer</code>的主要任务不是对IO进行处理，而更多的负责对注册到<code>EventGroup</code>的<code>Channel</code>进行 init 处理，其中大多是进行加入 Handler 的处理</p>
<h4 id="Ⅱ-ChannelOutboundHandler接口"><a href="#Ⅱ-ChannelOutboundHandler接口" class="headerlink" title="Ⅱ ChannelOutboundHandler接口"></a>Ⅱ ChannelOutboundHandler接口</h4><p>出站数据将由<code>ChannelOutboundHandler</code>处理，它的方法将被<code>Channel</code>，<code>ChannelPipeline</code>以及<code>ChannelHandlerContext</code>调用 （<code>Channel</code>，<code>ChannelPipeline</code>，<code>ChannelHandlerContext</code>都拥有<code>write</code>操作），以下是<code>ChannelOutboundHandler</code>的主要方法：</p>
<table>
<thead>
<tr>
<th align="left">状态</th>
<th align="left">描述</th>
</tr>
</thead>
<tbody><tr>
<td align="left">bind</td>
<td align="left">当Channel绑定到本地address时会调用此方法</td>
</tr>
<tr>
<td align="left">connect</td>
<td align="left">当Channel连接到远程节点时会调用此方法</td>
</tr>
<tr>
<td align="left">disconnect</td>
<td align="left">当Channel和远程节点断开时会调用此方法</td>
</tr>
<tr>
<td align="left">close</td>
<td align="left">当关闭Channel时会调用此方法</td>
</tr>
<tr>
<td align="left">deregister</td>
<td align="left">当Channel从它的EventLoop注销时会调用此方法</td>
</tr>
<tr>
<td align="left">read</td>
<td align="left">当从Channel读取数据时会调用此方法</td>
</tr>
<tr>
<td align="left">flush</td>
<td align="left">当Channel将数据冲刷到远程节点时调用此方法</td>
</tr>
<tr>
<td align="left">write</td>
<td align="left">当通过Channel将数据写入到远程节点时调用此方法</td>
</tr>
</tbody></table>
<p><strong>几乎所有的方法都将 <code>ChannelPromise </code>作为参数</strong>, 一旦请求结束要通过 <code>ChannelPipeline </code>转发的时候，必须通知此参数。</p>
<p><strong>ChannelPromise vs. ChannelFuture</strong>：</p>
<ul>
<li><code>ChannelPromise </code>是 特殊的 <code>ChannelFuture</code> <strong>（<code>ChannelPromise </code>扩展了 <code>ChannelFuture</code>）</strong>。任何时候调用例如 <code>Channel.write(...)</code> 一个新的 <code>ChannelPromise</code>将会创建并且通过 <code>ChannelPipeline</code>传递。这次写操作本身将会返回 <code>ChannelFuture</code>， 这样只允许你得到一次操作完成的通知。Netty 本身使用 <code>ChannelPromise </code>作为返回的 <code>ChannelFuture</code> 的通知，事实上在大多数时候就是 <code>ChannelPromise </code>自身</li>
</ul>
<h3 id="④-资源管理"><a href="#④-资源管理" class="headerlink" title="④ 资源管理"></a>④ 资源管理</h3><p><strong>当我们使用<code>ChannelInboundHandler</code>的<code>read</code>或<code>ChannelOutboundHandler</code>的<code>write</code>操作时，我们都需要保证 没有任何资源泄露并尽可能的减少资源耗费。</strong></p>
<p>之前已经介绍过了<code>ReferenceCount</code>引用计数用于处理池化的 <code>ByteBuf</code>资源。 为了帮助我们诊断潜在的的资源泄露问题，Netty提供了<code>ResourceLeakDetector</code>，它将 对我们的Netty程序的已分配的缓冲区做大约 1% 的采样用以检测内存泄露，Netty 目前定义了4种泄露检测级别，如下</p>
<table>
<thead>
<tr>
<th align="left">级别</th>
<th align="left">描述</th>
</tr>
</thead>
<tbody><tr>
<td align="left">Disabled</td>
<td align="left">禁用泄露检测。我们应当在详细测试之后才应该使用此级别。</td>
</tr>
<tr>
<td align="left">SIMPLE</td>
<td align="left">使用1%的默认采样率检测并报告任何发现的泄露，这是默认的检测级别。</td>
</tr>
<tr>
<td align="left">ADVANCED</td>
<td align="left">使用默认的采样率，报告任何发现的泄露以及对应的消息的位置。</td>
</tr>
<tr>
<td align="left">PARANOID</td>
<td align="left">类似于ADVANCED，但是每次都会对消息的访问进行采样，此级别可能会对程序的性能造成影响，应该用于调试阶段。</td>
</tr>
</tbody></table>
<p>我们可以通过 JVM 启动参数来设置<code>leakDetector</code>的级别：</p>
<pre class="line-numbers language-text" data-language="text"><code class="language-text">java -Dio.netty.leakDetectionLevel=ADVANCED<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<h2 id="2-ChannelPipeline"><a href="#2-ChannelPipeline" class="headerlink" title="2. ChannelPipeline"></a>2. ChannelPipeline</h2><p>在 Netty 组件中也介绍过了，<code>ChannelPipeline</code>是一系列<code>ChannelHandler</code>组成的拦截链，每一个新创建的<code>Channel </code>都会被分配一个新的<code>ChannelPipeline</code>，<code>Channel</code>和<code>ChannelPipeline</code>之间的关联是持久的，无需我们干涉它们 之间的关系。</p>
<h3 id="①-ChannelPipeline-相对论"><a href="#①-ChannelPipeline-相对论" class="headerlink" title="① ChannelPipeline 相对论"></a>① ChannelPipeline 相对论</h3><p>Netty 总是将<code>ChannelPipeline</code>的入站口作为头部，出站口作为尾部，当我们通过<code>ChannelPipeline</code>的<code>add</code>方法 将入站处理器和出站处理器混合添加到<code>ChannelPipeline</code>后，<code>ChannelHandler</code>的顺序如下：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115624803.png" alt="image-20220327115624803" style="zoom:80%;" />

<p>一个入站事件将从<code>ChannelPipeline</code>的头部（左侧）向尾部（右侧）开始传播，出站事件的传播则是与入站的传播方向相反。当<code>ChannelPipeline</code>在<code>ChannelHandler</code>之间传播事件的时候，它会判断下一个<code>ChannelHandler</code>的类型是否与当前<code>ChannelHandler</code>的类型相同（通过 <code>ChannelHandlerContext</code>），如果相同则说明它们是一个方向的事件， 如果不同则跳过该<code>ChannelHandler</code>并前进到下一个<code>ChannelHandler</code>，直到它找到相同类型的<code>ChannelHandler</code>。</p>
<h3 id="②-修改ChannelPipeline"><a href="#②-修改ChannelPipeline" class="headerlink" title="② 修改ChannelPipeline"></a>② 修改ChannelPipeline</h3><p><code>ChannelPipeline</code>可以通过添加，删除和修改<code>ChannelHandler</code>来修改它自身的布局，这是它最基本的能力， 一下列举了<code>ChannelPipeline</code>的一些修改方法：</p>
<table>
<thead>
<tr>
<th>名称</th>
<th>描述</th>
</tr>
</thead>
<tbody><tr>
<td>addFirst addBefore addAfter addLast</td>
<td>添加 ChannelHandler 到 ChannelPipeline.</td>
</tr>
<tr>
<td>Remove</td>
<td>从 ChannelPipeline 移除 ChannelHandler.</td>
</tr>
<tr>
<td>Replace</td>
<td>在 ChannelPipeline 替换另外一个 ChannelHandler</td>
</tr>
</tbody></table>
<h3 id="③-ChannelHandler的执行和阻塞"><a href="#③-ChannelHandler的执行和阻塞" class="headerlink" title="③ ChannelHandler的执行和阻塞"></a>③ ChannelHandler的执行和阻塞</h3><p>通常<code>ChannelPipeline</code>中的每个<code>ChannelHandler</code>都是通过它（<code>ChannelPipeline</code>）的<code>EventLoop</code>线程来处理 传递给他的数据的，所以我们不能去阻塞这个线程，否则会对整体的 IO 操作产生负面影响。 但有时候不得已 需要使用阻塞的 API 来完成逻辑处理，对于这种情况，<code>ChannelPipeline</code>的某些方法支持接受一个<code>EventLoopGroup </code>类型的参数，我们可以通过自定义<code>EventLoopGroup</code>的方式，使<code>ChannelHandler</code>在我们的<code>EventLoopGroup</code>内处理数据。 这样一来，就可以避免阻塞线程的影响了。</p>
<h3 id="④-触发事件"><a href="#④-触发事件" class="headerlink" title="④ 触发事件"></a>④ 触发事件</h3><p><code>ChannelPipeline</code>的API不仅有对<code>ChannelHandler</code>的增删改操作，还有对入站和出站操作的附加方法，如下：</p>
<p><code>ChannelPipeline</code>的入站方法：</p>
<table>
<thead>
<tr>
<th align="left">方法</th>
<th align="left">描述</th>
</tr>
</thead>
<tbody><tr>
<td align="left">fireChannelRegistered</td>
<td align="left">调用ChannelPipeline中下一个ChannelInboundHandler的channelRegistered方法</td>
</tr>
<tr>
<td align="left">fireChannelUnregistered</td>
<td align="left">调用ChannelPipeline中下一个ChannelInboundHandler的channelUnregistered方法</td>
</tr>
<tr>
<td align="left">fireChannelActive</td>
<td align="left">调用ChannelPipeline中下一个ChannelInboundHandler的channelActive方法</td>
</tr>
<tr>
<td align="left">fireChannelInactive</td>
<td align="left">调用ChannelPipeline中下一个ChannelInboundHandler的channelInactive方法</td>
</tr>
<tr>
<td align="left">fireExceptionCaught</td>
<td align="left">调用ChannelPipeline中下一个ChannelInboundHandler的exceptionCaught方法</td>
</tr>
<tr>
<td align="left">fireUserEventTriggered</td>
<td align="left">调用ChannelPipeline中下一个ChannelInboundHandler的userEventTriggered方法</td>
</tr>
<tr>
<td align="left">fireChannelRead</td>
<td align="left">调用ChannelPipeline中下一个ChannelInboundHandler的channelRead方法</td>
</tr>
<tr>
<td align="left">fireChannelReadComplete</td>
<td align="left">调用ChannelPipeline中下一个ChannelInboundHandler的channelReadComplete方法</td>
</tr>
<tr>
<td align="left">fireChannelWritabilityChanged</td>
<td align="left">调用ChannelPipeline中下一个ChannelInboundHandler的channelWritabilityChanged方法</td>
</tr>
</tbody></table>
<p><code>ChannelPipeline</code>的出站方法：</p>
<table>
<thead>
<tr>
<th align="left">方法</th>
<th align="left">描述</th>
</tr>
</thead>
<tbody><tr>
<td align="left">bind</td>
<td align="left">调用ChannelPipeline中下一个ChannelOutboundHandler的bind方法，将Channel与本地地址绑定</td>
</tr>
<tr>
<td align="left">connect</td>
<td align="left">调用ChannelPipeline中下一个ChannelOutboundHandler的connect方法，将Channel连接到远程节点</td>
</tr>
<tr>
<td align="left">disconnect</td>
<td align="left">调用ChannelPipeline中下一个ChannelOutboundHandler的disconnect方法，将Channel与远程连接断开</td>
</tr>
<tr>
<td align="left">close</td>
<td align="left">调用ChannelPipeline中下一个ChannelOutboundHandler的close方法，将Channel关闭</td>
</tr>
<tr>
<td align="left">deregister</td>
<td align="left">调用ChannelPipeline中下一个ChannelOutboundHandler的deregister方法，将Channel从其对应的EventLoop注销</td>
</tr>
<tr>
<td align="left">flush</td>
<td align="left">调用ChannelPipeline中下一个ChannelOutboundHandler的flush方法，将Channel的数据冲刷到远程节点</td>
</tr>
<tr>
<td align="left">write</td>
<td align="left">调用ChannelPipeline中下一个ChannelOutboundHandler的write方法，将数据写入Channel</td>
</tr>
<tr>
<td align="left">writeAndFlush</td>
<td align="left">先调用write方法，然后调用flush方法，将数据写入并刷回远程节点</td>
</tr>
<tr>
<td align="left">read</td>
<td align="left">调用ChannelPipeline中下一个ChannelOutboundHandler的read方法，从Channel中读取数据</td>
</tr>
</tbody></table>
<h2 id="3-ChannelHandlerContext"><a href="#3-ChannelHandlerContext" class="headerlink" title="3. ChannelHandlerContext"></a>3. ChannelHandlerContext</h2><h3 id="①-ChannelHandlerContext基本用法"><a href="#①-ChannelHandlerContext基本用法" class="headerlink" title="① ChannelHandlerContext基本用法"></a>① ChannelHandlerContext基本用法</h3><p><code>ChannelHandlerContext</code>代表的是<code>ChannelHandler</code>和<code>ChannelPipeline</code>之间的关联，每当有<code>ChannelHandler </code>添加到<code>ChannelPipeline</code>中时，都会创建<code>ChannelHandlerContext</code>。**<code>ChannelHandlerContext</code>的主要功能是 管理它所关联的<code>ChannelHandler</code>与同一个<code>ChannelPipeline</code>中的其他<code>ChannelHandler</code>之间的交互**：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115633870.png" alt="image-20220327115633870" style="zoom:80%;" />

<p><code>ChannelHandlerContext</code>的大部分方法和<code>Channel</code>和<code>ChannelPipeline</code>相似，但有一个重要的区别是： 调用<code>Channel</code>或<code>ChannelPipeline</code>的方法其影响是会沿着整个 <code>ChannelPipeline</code> 进行传播：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token comment">//使用Chanel write</span>
<span class="token class-name">Channel</span> channel <span class="token operator">=</span> ctx<span class="token punctuation">.</span><span class="token function">channel</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
ctx<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>xxx<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">//使用Pipeline write</span>
<span class="token class-name">ChannelPipeline</span> pipeline <span class="token operator">=</span> ctx<span class="token punctuation">.</span><span class="token function">pipeline</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
pipeline<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>xxx<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115640046.png" alt="image-20220327115640046" style="zoom:80%;" />

<p>而调用 <code>ChannelHandlerContext</code> 的方法则是从其关联的 <code>ChannelHandler</code> 开始，并且只会传播给位于该<code>ChannelPipeline</code>中的下一个能够处理该事件的 <code>ChannelHandler</code>：</p>
<pre class="line-numbers language-JAVA" data-language="JAVA"><code class="language-JAVA">&#x2F;&#x2F;使用ChannelContext write
ChannelHandlerContext ctx &#x3D; context;
ctx.write(xxx);<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115646138.png" alt="image-20220327115646138" style="zoom:80%;" />

<p>下面是一些比较重要的方法，有些和<code>ChannelPipeline</code>功能相似的方法就不再罗列了，各位同学可以直接查看原API。</p>
<table>
<thead>
<tr>
<th align="left">方法</th>
<th align="left">描述</th>
</tr>
</thead>
<tbody><tr>
<td align="left">alloc</td>
<td align="left">获取与当前ChannelHandlerContext所关联的Channel的ByteBufAllocator</td>
</tr>
<tr>
<td align="left">handler</td>
<td align="left">返回与当前ChannelHandlerContext绑定的ChannelHandler</td>
</tr>
<tr>
<td align="left">pipeline</td>
<td align="left">返回与当前ChannelHandlerContext关联的ChannelPipeline</td>
</tr>
<tr>
<td align="left">…</td>
<td align="left">…</td>
</tr>
</tbody></table>
<h3 id="②-ChannelHandlerContext-的高级用法"><a href="#②-ChannelHandlerContext-的高级用法" class="headerlink" title="② ChannelHandlerContext 的高级用法"></a>② ChannelHandlerContext 的高级用法</h3><p>有时候我们需要在多个<code>ChannelPipeline</code>之间共享一个<code>ChannelHandler</code>，以此实现<strong>跨管道处理（获取）数据</strong> 的功能，此时的<code>ChannelHandler</code>属于多个<code>ChannelPipeline</code>，且会绑定到不同的<code>ChannelHandlerContext</code>上。 </p>
<p><strong>在多个<code>ChannelPipeline</code>之间共享<code>ChannelHandler</code>我们需要使用 <code>@Sharable</code> 注解</strong>，这代表着它是一个共享的 <code>ChannelHandler</code>，如果一个<code>ChannelHandler</code>没有使用<code>@Sharable</code>注解却被用于多个<code>ChannelPipeline</code>，那么 将会触发异常。 还有非常重要的一点：<strong>一个<code>ChannelHandler</code>被用于多个<code>ChannelPipeline</code>肯定涉及到多线程 数据共享的问题，因此我们需要保证<code>ChannelHandler</code>的方法同步。</strong> 下面是一个很好的例子：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token annotation punctuation">@ChannelHandler.Sharable</span>    
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">UnsafeSharableChannelHandler</span> <span class="token keyword">extends</span> <span class="token class-name">ChannelInboundHandlerAdapter</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">private</span> <span class="token keyword">int</span> count<span class="token punctuation">;</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">channelRead</span><span class="token punctuation">(</span><span class="token class-name">ChannelHandlerContext</span> ctx<span class="token punctuation">,</span><span class="token class-name">Object</span> msg<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        count<span class="token operator">++</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"count : "</span> <span class="token operator">+</span> count<span class="token punctuation">)</span><span class="token punctuation">;</span>
        ctx<span class="token punctuation">.</span><span class="token function">fireChannelRead</span><span class="token punctuation">(</span>msg<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>上面这个<code>ChannelHandler</code>标识了<code>@Sharable</code>注解，这代表它需要被用于多个<code>ChannelPipeline</code>之间</p>
<p><strong>这段代码的问题是它持有状态: 一个实例变量 <code>count</code> 保持了方法调用的计数</strong>。将这个类的一个实例添加到 <code>ChannelPipeline</code> 并发访问通道时很可能产生错误。一个简单的解决方法就是给修改了<code>count</code>的变量的方法 <code>channelRead</code> 加<code>synchronized</code>关键字，确保即使在多个<code>ChannelPipeline</code>之间共享， <code>ChannelHandler</code>也能保证数据一致。</p>
<h2 id="引用"><a href="#引用" class="headerlink" title="引用"></a>引用</h2><blockquote>
<p>《Netty 实战》</p>
</blockquote>

                
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